1. Field of Invention
The present invention is intended to transmit ions from higher to lower pressure regions such as atmospheric pressure interfacing of ionization source to vacuum mass spectrometry or ion mobility spectrometry.
2. Description of Prior Art
Dispersive sources of ions at or near atmospheric pressure; such as, atmospheric pressure discharge ionization, chemical ionization, photoionization, or matrix assisted laser desorption ionization, and electrospray ionization generally have low sampling efficiency through conductance or transmission apertures, where less than 1% [often less than 1 ion in 10,000] of the ion current emanating from the ion source make it into the lower pressure regions of the present commercial interfaces for mass spectrometry.
Fenn, et al. (1985) U.S. Pat. No. 4,542,293 demonstrated the utility of utilizing a dielectric capillary to transport gas-phase ions from atmospheric pressure to low pressure where the viscous forces within a capillary push the ions against a potential gradient. This technology has the significant benefit of allowing grounded needles with electrospray sources. Unfortunately, this mainstream commercial technology transmits only a fraction of a percent of typical atmospheric pressure generated ions into the vacuum. The majority of ions being lost at the inlet due to dispersive fields dominating the motions of ions (FIG. 8). The requirement of capacitive charging of the tube for stable transmission, as well as, transmission being highly dependent on surface charging creates limitations on efficiencies with this technology. Contamination from condensation, ion deposition, and particulate materials can change the surface properties and the transmission. Because of the large surface area contained on the inner wall surface, a large amount of energy is stored and can discharge and damage the electrode surfaces. Care must also be taken to keep the outer surfaces clean and unobstructed, presumably in order not to deplete the image current that flows on the outer surface of the dielectric.
Chowdhury, et al. (1990) U.S. Pat. No. 4,977,320 demonstrated the use of heated metal capillaries to both generate and transmit ions into the vacuum. The efficiencies of this device are low as well. This technology samples both ions and charged droplets into the capillary where, with the addition of heat, ion desorption is facilitated. Undergoing coulomb explosions inside of a restricted volume of the tube will tend to cause dispersion losses to walls with this technique. In addition, this technique encounters the same limitation from dispersion losses at the inlet as the dielectric capillaries.
Lin and Sunner (1994) (J. American Society of Mass Spectrometry, Vol. 5, Number 10, pp. 873-885, October 1994) study a variety of effects on transmission through tubes of glass, metal, and Teflon. A wide variety of parameters were studied including capillary length, gas throughput, capillary diameter, and ion residence time. Effects from space charge, diffusion, gas flow, turbulence, spacing, and temperature where evaluated. These studies failed to identify the field dispersion at the inlet as the major loss mechanism for ions in capillaries. Some important insights where reported with respect to general transmission characteristics of capillary inlets.
Franzen (1998) U.S. Pat. No. 5,736,740 proposes the use of weakly conducting inner surfaces to prevent charge accumulation as a means to facilitate the focusing of ions toward the axis of the capillary. Although it is difficult to distinguish this art from Fenn in that the glass tubes utilized in commercial applications under Fenn also utilize weakly conducting dielectric surfaces, Franzen does argue effectively for the need to control the inner surface properties and the internal electric fields. This device will suffer from the same limitations as Fenn.
Franzen (1998) U.S. Pat. No. 5,747,799 also proposes for the need to focus ions at the inlet of capillaries and apertures in order enhance collection efficiencies. In this device the ions are said to be entrained into the flow by viscous friction. This invention fails to account for the dominance of the electric field on the motion of ions in the entrance region. At typical flow velocities at the entrance of tubes or apertures, the electric fields will dominate the ion motion and the ions that are not near the capillary axis will tend to disperse and be lost on the walls of the capillary or aperture inlet. With this device, a higher ion population can be presented to the conductance opening at the expense of higher field ratios and higher dispersion losses inside the tube.
Forssmann, et al. (2002) WO 03/010794 A2 utilizes funnel optics in front of an electrospray source in order to concentrate ions on an axis of flow by imposing focusing electrodes of higher electrical potential than the bottom of the so called accelerator device. This device frankly will not work. The ions formed by the electrospray process will be repelled by this optics configuration and little to no transmission will occur. Most of the inertial energy acquired by the ions in the source region is lost to collisions with neutral gas molecules at atmospheric pressure; consequently the only energy driving the ions in the direction of the conductance aperture will be the gas flow which under normal gas flows would be insufficient to push the ions up a field gradient. This device does not operate in fully developed flow as will be described in the present invention.
Fischer, et al. (2002) U.S. Pat. No. 6,359,275 B1 address the issue of charging of the inner surface of the capillary by coating the inner surface with a conductor in the dispersive region of the tube while still keeping the benefits of the dielectric tube transport in the nondispersive region of the capillary. This approach addresses the problem of charge accumulation, but it does not remove the significant losses due to dispersion at the inlet.
Fischer, et al. (2002) U.S. Pat. No. 6,486,469 B1 utilizes external electrodes and butted capillary tubes to provide enhanced control of the electric field within the capillary. This device does not address issues related to inlet losses as presented in FIG. 1. In addition, the device still required significantly large dielectric surfaces with the associated problems with charging, contamination, and discharge.
Fischer, et al. (2003) U.S. Patent Application US 2003/003452 A1 and Fischer, et al. (2003) U.S. Pat. No. 6,583,407 B1 utilized a variety of modifications to their dielectric tube device to enhance selectivity and control of ions as they traverse their capillary device. None of these modifications addresses the aforementioned limitations of these capillary devices.
U.S. Pat. No. 6,455,846 B1 to Prior et al. (2002) discloses a flared or horn inlet for introducing ions from an atmospheric ionization chamber into the vacuum chamber of a mass spectrometer. They also reported that the increase in ion current recorded in the mass spectrometer was directly proportional to the increase in the opening of the flared inlet.
U.S. Pat. No. 6,583,408 B2 to Smith et al. (2003) has recently utilized multi-capillary arrays as an inlet to their ion funnel technology. This device reports an advantage of bundle tubes over single opening conductance pathways, but fails to address the major issue relating to ion transmission loss, namely field dispersion of ions at the entrance of the conductance opening. A bundle of tubes without controlled field throughout the conductance path will still have significant losses when sampling higher field sources.
Ion movement at higher pressures is not governed by the ion-optical laws used to describe the movement of ions at lower pressures. At lower pressures, the mass of the ions and the influence of inertia on their movement play a prominent role. While at higher pressures the migration of ions in an electrical field is constantly impeded by collisions with the gas molecules. In essence at atmospheric pressure there is so many collisions that the ions have no “memory” of previous collisions and the initial energy of the ion is “forgotten”. Their movement is determined by the direction of the electrical field lines and the viscous flow of gases. At low viscous gas flow, the ions follow the electric field lines, while at higher viscous gas flow the movement is in the direction of the gas flow. Inventors have disclosed various means of moving ions at atmospheric pressure by shaping the electric field lines and directing the flow of gases. FIG. 8 is a simulation of ion trajectories under forces of both electric field and flow. Experimental evidence and theory support the premise that the electric field dominated the motion of ions in the entrance region of most high field sources where ions are focused at the conductance aperture.
Our co-pending U.S. Patent Application 60/419,699 (2003) describes the use of laminated tubes an apertures to control both field and flow in the entire conductance pathway from the entrance to the exit. Delaying dispersion until flow has fully developed is described in this patent as a technique to minimize dispersion losses within the conductance pathway. FIG. 9 illustrates the typical flow development within a laminar flow tube. FIG. 10 illustrates the lack of dispersion when laminated tubes are utilized to maintain uniform field throughout the tube. The principals and methods of this patent are applied to the present invention where our laminated arrays operate with the same ion transmission advantage as observed with laminate tubes. Components of this invention are included by reference into the present invention.